hedonist222 I am under the assumption that group head pressure cannot exceed pump pressure?
In a rotary pumped machine it can, because the rotary pump pushes through a one-way valve into a sealed brew circuit. This means that although the pumps balanced bypass could be set to a pressure of 9 bar, when the water enters the sealed brew circuit it’s cold, as that water heats it expands further, increasing the pressure above 9 bar. This only shows when using a blind filter, because the expansion is quite small and there is no way for the pressure to escape. When doing a coffee shot, in reality there is no increasing pressure because the puck is permeable enough that any expansion releases through the puck.